Hydroprocessing reactions involve the application of hydrogen to a substrate, usually under elevated temperature and pressure, in the presence of a catalyst with the aim of causing a physical or chemical change in the substrate. Most such hydroprocessing reactions occur in refinery operations where the substrate is a hydrocarbon feedstock.
Conventional hydroprocessing catalysts are generally in the form of a carrier of a refractory oxide material on which hydrogenation metals are deposited, the choice and amount of each component being determined by the end use. Refractory oxide materials usual in the art are amorphous or crystalline forms of alumina, silica and combinations thereof. These oxide materials can have some intrinsic catalytic activity but often only provide the support on which active metals compounds are held. The metals are generally base or noble metals from Group VIII and Group VIB of the Periodic Table which are deposited in oxidic form during manufacture; in the case of base metals, the oxides are then sulphided prior to use to enhance their activity.
The use of titania, or titanium dioxide, as a catalyst support for a conventional hydroprocessing catalyst is limited by the lack of a useful pore structure. Therefore the few titania-supported commercial hydroprocessing catalysts that exist in the market have a low pore volume and as a result can hold or support less hydrogenation metals than the more common alumina-supported catalysts. Generally it is viewed that thermal stability, low surface area and poor mechanical strength have all hindered the commercial exploitation of titania supported catalyst systems. The intrinsic activity of hydrogenation metals-on-titania is, however, superior to eg alumina-based catalysts. The proposals available in the art attempt to harness this intrinsic activity and remedy the deficiencies of low metals loadings and thermal instability by using mixed oxides.
M. Breysse et al in Catalysis Today 86 (2003) 5-16, notes that the molybdenum loading on a typical titania supported system is generally limited to 6 wt % Mo because of the low surface area of the support but with recent improvements in preparing mesoporous titania this can be expected to increase to 10 to 12 wt %. Tests using a typical hydrogenation metal combination of nickel and molybdenum showed that a NiMo-titania catalyst had the lowest activity for tetralin conversion in the presence of H2S than NiMo on various mixed titania-alumina supports, and NiMo-alumina catalysts. Later in the same review article it is concluded that the presence of nickel or cobalt suppresses the higher intrinsic activity of molybdenum-titania systems.
G. M. Dhar et al. in Catalysis Today 86 (2003), 45-60, also looks at various mixed alumina-titania supported systems; hydrogenation metals are applied by the conventional incipient wetness impregnation method and an improved HDS and hydrogenation activity is attributed to increased metals dispersion. Here the presence of small amounts (3 wt %) of nickel and cobalt are considered to promote, eg, HDS activity of a catalyst of 8 wt % molybdenum on mixed titania-alumina supports. In a study of variation of Mo loading, the maximum molybdenum content considered is 14 wt % (as the oxide and basis total catalyst).
Also proposed in the art for hydrotreating and particularly for use in hydrodesulfurization (HDS), especially deep desulfurisation of diesel fractions, are catalyst compositions which contain refractory oxide material but which are made via co-precipitation. European Patent specification EP-A-1090682 describes one such co-precipitation proposal to prepare a hydrotreating catalyst, which catalyst has various properties including a crystalline phase, such as alpha-alumina, viewed as necessary for high activity and to impart mechanical strength and therefore a longer service life in commercial use.
By co-precipitation, the incorporation of a dispersed metals content into a conventional carrier material is attempted by enabling intimate contact between metals compounds and carrier material and thus enabling the metals to become dispersed through the carrier material before shaping. This contrasts with conventional impregnation techniques where only a small amount of metals deposition is possible since the shaped carrier is already formed and there are diffusional and space limitations for the metal ions or compounds to become dispersed through the catalyst support.
Alternative catalyst forms have been proposed for use in the hydroprocessing of, for example, refinery streams. One such group of catalysts are termed ‘bulk catalysts’. Such catalysts are formed from metal compounds only, usually by co-precipitation techniques, and have no need for a catalyst carrier or support; see for example WO 00/42119, U.S. Pat. No. 6,162,350 and WO 00/41810. These publications disclose bulk Group VIII and Group VIb metal catalysts and their preparation and use. U.S. Pat. No. 6,162,350 discloses that such catalysts may contain one or more of each metal type, and examples show NiMo, NiW and the most preferred NiMoW bulk catalysts. The preference in U.S. Pat. No. 6,162,350, WO 00/42119 and WO 00/41810 is that no binder is incorporated into the final catalyst composition since the activity of the bulk catalyst composition may be reduced (U.S. Pat. No. 6,162,350, Column 14, lines 10 to 114). If, however, a binder is to be used the resulting catalyst composition comprises the bulk catalyst particles embedded in the binder with the morphology of the bulk catalyst particles essentially maintained in the resulting catalyst composition (U.S. Pat. No. 6,162,35, Col. 14, lines 24 to 30). The binder when present is preferably added prior to shaping but can be added at any stage in the catalyst preparation.
The use of titania as a refractory oxide material or binder is proposed as one of many suitable oxide materials in these patent publications, but there is no indication that its use is actually contemplated or expected to provide any benefit over the alumina- and silica-bound forms exemplified.
In refinery processes, feedstocks contain a variety of contaminants, the main ones being sulfur and nitrogen. While sulfur reduction has always been desirable, increasingly strict regulations on gas emissions eg from motor vehicles, is driving the need for catalysts which can provide ultra low sulfur fuels. For effective HDS activity, and especially for the deep desulfurisation required for environmental reasons, a catalyst must be effective to remove all sulfur compounds, whether simple or complex. Nitrogen contaminants, while often low in amount, can have a severe poisoning effect on catalysts and also adversely affect end product storage stability and quality. The poisoning effect on catalysts is such that a catalyst effective for, eg HDS, of a pure chemical feedstock may be ineffective or short-lived when exposed to an impure refinery feedstock.
Thus, there is a continuing demand for hydroprocessing catalysts for feedstocks having both sulfur and nitrogen contaminants, which catalysts have a significant hydrodesulphurisation activity for both simple and complex sulfur-containing compounds in the presence of nitrogen contaminants but even more desirably also have a high or improved hydrodenitrogenation (HDN) activity.